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See my followup message https://forums.macrumors.com/threads/apple-releases-macos-tahoe-26-3.2477549/post-34445665 for a workaround that, at least, makes me feel that I am slightly in control 😉

Thanks for the reply, if only to reassure me that it was not something I was doing or a portent of imminent hardware catastrophe 😮
I suspect the update process to 26.3 rather than 26.3, in and of itself, was the problem. I eventually did a full power cycle, and every thing is back to normal. Copying from DMGs is very fast, Quick View on JPGs and PNGs, which had become glacial, is instant again.
 
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